WO2004008405A1

WO2004008405A1 - Below ground security system

Info

Publication number: WO2004008405A1
Application number: PCT/AU2003/000389
Authority: WO
Inventors: Donald Jaffrey
Original assignee: Future Fibre Technologies Pty Ltd
Priority date: 2002-07-17
Filing date: 2003-03-31
Publication date: 2004-01-22
Also published as: AU2002950232A0

Abstract

Below ground security systems are disclosed which include at least one first waveguide (10). The waveguide (10) is buried below ground level and the fill above the cable preferably comprises gravel or pebble fill (G) above the waveguide. The waveguide may be in the form of a micro-strain sensor. Various configurations are disclosed which include additional waveguides (12) arranged in a straight line configuration, or waveguides (38, 36) arranged in a loop configuration, or waveguides (36, 38, 47) arranged in an overlapping loop configuration, with the waveguide (47) being above the waveguides (36, 38). An additional motion vibration sensor waveguide (40) may also be included.

Description

BELOW GROUND SECURITY SYSTEM

Field of the Invention

This invention relates to a below ground security system in which optical fibre waveguides are laid below ground and provide a sensor for detecting movement over the ground to provide an alarm condition indicative of an intrusion into or departure from a secure area.

Background Art

Our co-pending International Application No. PCT/AU02/00007 discloses a below ground system for providing a security perimeter. This system discloses cables which are laid in a trench and which are covered by soil. The cables are laid in a zigzagged pattern and the number of cables and waveguides within those cables not only provide detection of travel across the secure region, but also the location of that breach. The contents of this International application are incorporated into this specification by this reference.

The object of the present invention is to further improve the configuration of below ground systems to provide cost effective systems, as well as providing greater sensitivity, for use in various locations depending on the security risk involved.

Summary of the Invention

The invention, in a first aspect, may be said to reside in a below ground security system including: a first cable having at least one first waveguide which forms at least part of a sensor configuration for detecting a breach of the security system; the first waveguide being buried below ground level; and a fill above the cable, and beneath which the cable is buried, the fill comprising a gravel or pebble fill .

It has been found that the use of a gravel or pebble fill which is deposited on top of the waveguide, provides increased transmission of vibrations to the waveguide, thereby making it easier for the waveguides to detect a breach across the perimeter defined by the waveguides. Thus, breaches across the secured perimeter are more easily detected.

In the preferred form of the invention, the system includes a second cable having at least one second waveguide, the first waveguide and second waveguide forming arms of a micro-strain sensor configuration.

In another embodiment of the invention, the at least one waveguide forms the sensor and operates in motion vibration sensor configuration.

In the first said embodiment, the first and second waveguides form two arms of a sensor so that a disturbance of one of the waveguides relative to the other changes a characteristic of light propagated through one of the waveguides, so as to change an interference pattern created when light from the first and second waveguides is recombined, to thereby provide an indication of a breach of the security system.

Preferably the system includes a light source for launching light into the waveguides, and a detector for detecting light from the waveguides.

Preferably the sensor is a micro-strain sensor. However, other forms of sensor could be used.

In one embodiment, at least three cables are provided, each cable having at least one waveguide. In one arrangement, one of the waveguides is a common waveguide and the two other waveguides comprise separate waveguides for providing two arms of a micro-strain sensor, with said one of the arms returning recombined light from both said two arms which form the micro-strain sensor.

In another embodiment, the at least two waveguides are laid in a zigzagged pattern relative to one another.

In a still further embodiment, a further cable including at least one waveguide is provided, the further cable forming a motion vibration sensor which is used alongside the micro-strain sensor.

This embodiment of the invention has particular application for use of the secure perimeter near environments in which high impact activity may take place. Such environments may include prisons and, in particular, exercise yards in which sports such as basketball or the like may be played. This arrangement has the advantage of enabling the first and second arms which form the micro- strain sensor to be laid away from the exercise area so that the good lateral sensitivity of the micro-strain sensor does not detect the vibrations caused by activity in the exercise area, so that the sensor is not activated by normal exercise in the exercise yard. The single motion vibration sensor can be laid in close proximity to the exercise yard because it has limited lateral sensitivity and, therefore, will not be triggered by normal activity in the exercise yard. However, the motion vibration sensor has good vertical sensitivity and therefore, should an attempt be made to scale a fence near the exercise yard and drop onto the ground, the motion vibration sensor which can be laid near the fence, will detect that attempted breach of the perimeter, and then any attempt to move away from the fence and across the two arms which form the micro-strain sensor will trigger the micro-strain sensor.

A further embodiment of the invention provides a pair of waveguides arranged in a curved or undulating pattern into which light is launched, and at least one return waveguide for returning light from the said pair of waveguides, the at least one return waveguide being laid below the depth of the said two waveguides.

In one embodiment of the invention, the at least one return waveguide is laid in a straight line. However, in another embodiment, the said at least one return waveguide is also laid in a curved or undulating pattern.

The invention also may be said to reside in a below ground security perimeter system, including: a first cable having at least one waveguide laid in a substantially straight line below ground level; a second cable having at least one waveguide laid in a substantially straight line below ground level; a light source for launching light into the first and second waveguides; at least one return waveguide for returning recombined light which has travelled through the first and second waveguides; and a detector for detecting light returned by the at least one return waveguide and for monitoring a parameter of the light to determine a breach of the security system.

In one embodiment, two return waveguides are provided.

Preferably the or each return waveguide is provided in a separate cable.

In one embodiment, the two return waveguides may form a continuation of the first and second waveguides so that the first and second waveguides form twin arms of a single micro-strain sensor.

However, in another embodiment, the end of the first and second waveguides and the at least one return waveguide remote from the light source are coupled by a coupler for combining the light after the light has been transmitted through the waveguides. This configuration provides twin micro-strain sensors.

This aspect of the invention has the advantage that the cables are laid in a straight line, which means that the cables are more easy to insert into the ground by simple trenching methods, and, because the cables are laid in a straight line, the cables can more readily be laid by machine. Thus, the cables are more easily laid and also for any particular length of the perimeter, this configuration provides the possibility of the most economical use of the waveguides.

The disadvantage of this configuration is that, because the waveguides are laid in a straight line, their sensitivity is not maximised because the sensitivity is somewhat dependent upon the length of the cable close to the source of the disturbance. Also, the possibility of null points in the system does exist, which means that if a person steps in a place equidistant from the two arms, detection by the arms of the micro-strain sensor may cancel each other out, thereby causing the alarm not to be given. The likelihood of this happening is extremely unlikely, but nevertheless does academically exist with this form of configuration.

However, this aspect of the invention does lend itself to relatively economical perimeter barrier systems which may be used in lower grade security installations.

The system also provides good lateral sensitivity and good vertical sensitivity because the two arm micro-strain sensor configuration used in this embodiment lends itself to detection of vibrations which occur at some distance from the actual location of the cables, as well as vibrations which occur directly above the cables. Thus, lateral and vertical sensitivity is good.

A further aspect of the invention provides a below ground security perimeter system, including: a first cable having a first waveguide; a second cable having a second waveguide, the first and second cables being laid in a curved configuration and forming a micro-strain sensor; a third cable having a third waveguide and forming a motion vibration sensor; the first, second and third cables being buried below ground level; a light source for launching light into the first, second and third waveguides; and a detector for detecting light received from the first, second and third waveguides, and for detecting a change in parameter of light launched into the waveguides to thereby detect a breach of the security perimeter.

This aspect of the invention provides a system which has both good lateral and vertical sensitivity which is provided by the first and second waveguides which are arranged in the curved configuration, because these waveguides can be used to form a two-arm micro-strain sensor in which light travelling in one arm is compared to light travelling in the other arm in order to determine a breach of the system. However, this arrangement also lends itself for use in environments where high impact may occur because the first and second waveguides can be located at a distance from the area of high impact, thereby not triggering the alarm because the micro-strain sensor having good lateral sensitivity is moved away from the high impact area. However, the motion vibration sensor which has very limited lateral sensitivity, can be located near the high impact area, but not triggered by the high impacts unless those impacts occur close to or directly vertically above the single motion vibration sensor. One environment in which this configuration has particular application is prisons in which the security perimeter barrier may be required in the vicinity of an exercise yard where high impacts will occur due to exercise in the exercise yard.

A further aspect of the invention provides a below ground security perimeter system, including: a first cable having a first waveguide; a second cable having a second waveguide; the first and second cables being laid in a curved or undulating configuration; a light source for launching light into the first and second waveguides; at least one third waveguide for returning recombined light which has travelled through the first and - second waveguides; a detector for detecting light returned by the return waveguide; the first, second and third waveguides being buried below ground level; and the first and second waveguides forming two arms of a micro-strain sensor.

The laying of the waveguides in the above aspects of the invention in a curved or undulating configuration provides much greater sensitivity because of the greater length of waveguide which is employed. However, the disadvantage is that the curved configuration cannot be so readily laid by a machine and therefore may need to be hand laid, which increases the cost of installation. The length of cable used is also increased. However, these configurations provide the possibility of the greatest sensitivity and therefore the most secure perimeter system.

In one embodiment of the invention, the return waveguide is laid in a straight line.

However, in another embodiment, the' return waveguide is also laid in a curved or undulating configuration similar to that of the first and second waveguides. This configuration also avoids the possibility of null points in the system.

Brief Description of the Drawings

Preferred embodiments of the invention will be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a plan view of an underground system according to the first embodiment of the invention;

Figure 2 is a cross-sectional view of the system of Figure 1, showing the cables laid beneath ground level;

Figures 3 and 4 are views similar to Figures 1 and 2, but of a second embodiment of the invention;

Figures 5 and 6 are views of a third embodiment of the invention; Figures 7 and 8 are views of a fourth embodiment of the invention;

Figures 9 and 10 are views of a fifth embodiment of the invention; and

Figures 11 and 12 are views of a sixth embodiment of the invention.

Detailed Description of the Preferred Embodiments With reference to Figure 1, an underground system is shown which comprises a first cable 10, a second cable 12, a third cable 14 and a fourth cable 16. Each of the cables 10 to 16 includes at least one fibre optic waveguide. In practice, such cables normally employed several waveguides, but only one of the waveguides in each cable need be used in this embodiment of the invention.

One end of the cables 10 to 16 are connected at a remote location to a junction box 20 which connects to a control station (not shown) . At the control station, a light source L such as a pigtail laser is provided for launching light into the cables 10 and 14, and a detector D is provided for detecting light which returns through the cables 12 and 16.

In the embodiment shown in Figure 1, the cable 12 is simply a continuation of the cable 10, and the cable 16 is continuation of the cable 14. Thus, light is launched into the cable 10 and simply travels along the cable 10 to junction box 22, and then the light returns along cable 12 which may be spliced to cable 10 (that is the waveguides in the cables) . Similarly, light launched into cable 14 travels along cable 14 and then returns along cable 16.

The cables 10 and 12 form one arm of a single micro-strain sensor and the cables 14 and 16 the other arm, so that if one of the arms is disturbed by someone traversing the perimeter barrier, the property of the light travelling through that arm changes, so that when the light returning along the cables 12 and 16 is recombined, the interference pattern caused by the recombination of the light changes to provide an indication of the breach of the perimeter.

Because this configuration of the invention operates in the form of a micro-strain sensor which has two arms, the system has extremely good lateral sensitivity in that a disturbance laterally of the cables 10 to 16, as well as a disturbance directly above the cables 10 to 16 can be sensed. Thus, the system provides good lateral and vertical sensitivity. In another aspect of this embodiment, the junction box 22 could include a coupler so that light could be launched through all four cables shown in Figure 1, and then recombine to return through a further return waveguide (not shown) and which could be provided by any waveguide in one of the cables 10 to 16. This configuration provides a twin sensor configuration in which effectively two micro-strain sensors each having two arms are then provided.

As is shown in Figure 2, the cables 10 to 16 are laid below ground level G and on a soil or gravel substrate S in a trench T. The trench T is then filled with gravel GR which may be in the form of pebbles or the like. If it is desired to provide a covert perimeter barrier system, the gravel can be covered by lawn or any other suitable material, so as to disguise the trench in which the cables 10 to 16 are laid. The use of the gravel above the cables 10 to 16 ensures that, if someone traverses the perimeter barrier, the vibrations which are caused by the slight movement of the gravel are more easily transmitted to the cables 10 to 16, so that detection of the breach of the perimeter barrier is more easily detected. The movement of the gravel itself provides additional vibrations due to the movement of the particles of gravel relative to one another, thereby making it virtually impossible for someone to traverse the cables 10 to 16 without triggering the alarm.

This embodiment of the invention also has the advantage that the cables 10 to 16 are laid in a straight line, thereby making the cables easy to lay by digging a trench and laying the cables by machine. The trench could be dug by tines mounted behind a tractor and the cable laid in those tine marks, and then filled with gravel.

Furthermore, the straight line configuration provides the possibility of using the minimum amount of cable. However, the disadvantage is that the sensitivity is somewhat dependent on the amount of cable per unit length of the perimeter, and therefore the straight line configuration therefore provides less sensitivity than the configuration in which more cable is used per unit length of perimeter barrier.

Thus, this embodiment provides relatively low installation costs, but does suffer from reduced sensitivity because of the straight line configuration.

This embodiment also suffers from the slight possibility that null points do exist in the system which, if a person steps at such a point, the waveguides of the micro-strain sensor detect exactly the same disturbances at the same instance, and therefore effectively cancel one another out, and this may result in the breach not being detected. Whilst a possibility of this happening is very small, it does nevertheless represent an academic possibility.

Figures 3 and 4 are similar to Figures 1 and 2, except only three cables are used. In this embodiment, this configuration provides even lower installation costs because only three cables are used instead of four.

In this embodiment, light is launched into one of the waveguides in cable 21 and cable 23. The light is transmitted along the waveguide to the end of the cables 21 and 23 which are coupled to a junction box 24. The junction box 24 includes a coupler for coupling the waveguides in the cables 21 and 23 to a waveguide in cable 26, so the light from the waveguides 20 and 22 is returned along the common waveguide in the cable 26 to junction box 28. Once again, the junction box 28 is coupled to a controller system 30 which includes a laser for launching the light into the cables 21 and 23 and a detector for detecting light returned by the cable 26. If a disturbance occurs in the vicinity of the cables 21 and 23, the light signal in the cable 21 is changed relative to cable 23 so that when the light in the two cables recombines before going into the cable 26, the change in interference pattern is produced to thereby provide an indication of a breach of the security perimeter.

This system provides good lateral sensitivity because of the micro-strain sensor configuration which includes two arms (provided by the waveguides in cables 21 and 23) and moderate vertical sensitivity because the configuration in this embodiment utilises three cables and therefore the width of the security perimeter is less than in the previous embodiment. This system also has null points, as in the previous embodiment. However, this arrangement provides the lowest installation cost and the lowest cable cost.

Figures 5 and 6 show a further embodiment which is similar to the embodiment described in the above-mentioned International application. As can be seen from Figure 5, cables 36 and 38 are laid in a looped or zigzagged type configuration and are joined at their ends at a junction box 39.

In this embodiment, light is launched into a waveguide in each of the cables 36 and 38 and recombined at a coupler in junction box 39 so as to travel back through one of the waveguides in the other of the cables 36 and 38 for detection. This embodiment provides excellent lateral sensitivity and good vertical sensitivity. However, because of the pattern in which the cables 36 and 38 are laid, it is generally necessary to hand lay the cable, which increases installation costs and the amount of cable used is obviously greater. However, the additional length of cable increases the lateral sensitivity of the system. Once again, the system does not exclude the possibility of null points.

Figures 7 and 8 show a further embodiment of the invention which is applicable to environments in which the perimeter barrier is likely to be installed in high impact areas such as prison yards and, in particular, exercise areas in prison yards. In such areas, activities such as basketball or the like may be undertaken, and this will result in significant impacts on the ground which may occur in proximity to a perimeter barrier. Because of the relatively high lateral sensitivity of a micro-strain configuration of the types previously described, those impacts are likely to be detected by the system and therefore false alarms will be generated. Thus, the micro-strain configuration needs to be deployed some distance from those high impact areas, which is a disadvantage if it is also desired to monitor a perimeter very close to the high impact area, such as on the outside of a fence or other barrier.

In order to overcome this problem, the embodiment of Figures 7 and 8 provide a micro-strain configuration which is generally the same as that described with reference to Figure 5, in which cables 38 and 36 are provided in a zigzagged or looped configuration, but are located some distance from the fence or barrier (not shown) which borders the exercise yard. Thus, high impacts in the exercise yard will not cause an alarm to be triggered. In order to monitor very close to the fence or barrier which surrounds the impact yard, a single cable 40 having at least one waveguide is laid and forms a motion vibration sensor cable. This cable has restricted lateral sensitivity and therefore will not be triggered by impacts in the exercise yard. However, the cable has good vertical sensitivity, so that any attempt to scale the fence or barrier from the exercise yard and drop down on the other side of the wall will trigger the motion vibration sensor when a person lands above the motion vibration sensor 40. Further security is provided by the fact that as soon as the person moves away from the fence towards the location of the cables 36 and 38, the cables 36 and 38 will then detect the breach of the barrier system and generate a further alarm. Thus, this embodiment provides a system which has very good vertical sensitivity because of the use of the vibration sensor cable 40, and very good lateral sensitivity, but spaced away from the vibration cable sensor so as not to be triggered by impacts which may legitimately occur in close proximity to the cable 40.

In this embodiment, the cable 40 is monitored by detecting light reflected from the end of the cable 40 and, in the event of a vibration applied to the cable 40, a characteristic of the light returning through the cable will be changed. That characteristic is detected to provide an indication of the breach of the security perimeter.

Again, the detection in the cables 36 and 38 occurs by the use of those cables in a micro-strain configuration in which light travelling along two paths is recombined, and a disturbance of one of the cables causes a change in interference pattern, which is detected by the detector.

Once again, in this embodiment, junction box 39 can be employed to enable various waveguides in the cables 36 and 38 to be combined so that the micro-strain sensor can be used as a single sensor having two arms, or a dual sensor by using more of the waveguides in each of the cables 36 and 38.

Figures 9 and 10 show a further embodiment in which a looped or curved configuration of cables 36 and 38 are used. However, in this embodiment, the loops do not overlap one another as in the embodiments of Figures 3 and 5. This embodiment has particular application if it is desired to provide a very wide perimeter barrier. The cables 36 and 38 are joined at junction box 39, which again can include couplers, so that light launched into the cables 36 and 38 is returned along straight cables 41 and 43. The cables 36 and 38 are connected to a junction box 45, as are the cables 41 and 43, and the junction box 45 is connected to controller 47, which includes the pigtail laser diode for launching light into the cables 36 and 38, and a detector for detecting return of light through the cables 41 and 43.

Once again, in this embodiment, a single micro-strain sensor can be used by using only one of the waveguides in the cables 36 and 38 and returning recombined light along one of the separate cables 41 and 43. Alternatively, by using multiple waveguides in the cables, a dual sensor configuration can be used.

This embodiment provides reduced cable installation costs because the cables 41 and 43 are laid in a straight line. However, in dual sensory configurations, because the length of the cables 36 and 38 needs to be the same as the return cables 41 and 43, this embodiment necessitates the coiling of a relatively large amount of cable at the vicinity of the junction box 45.

The cables 41 and 43, as is shown in Figure 10, are preferably buried deeper than the cables 36 and 38, and the coils at the end of the cables 41 and 43 are buried relatively deeply so as not to detect unwanted vibrations because of the large amount of cable which may be present in a relatively small volume, and which therefore provides a very sensitive sensor in its own right. The cables 41 and 43 can be machine laid because they are laid in a straight line. However, the cables 36 and 38 need to be hand laid. This configuration provides the highest lateral sensitivity and also the highest vertical sensitivity. However, once again, in a single sensory configuration, null points do exist in the system, and therefore there is a very small possibility that a person could traverse the system without triggering an alarm.

Figures 11 and 12 is an embodiment similar to Figure 9, except that only a single return cable 47 is used, and the cable 47 is laid in a curved or undulating pattern, the same as the cables 36 and 38. This embodiment therefore does away with the need to provide a coil of the cable near the junction box 45, because the configuration of the cables 36, 38 and 47 are the same, and therefore the length of the cables as laid in those configurations will be the same.

In this embodiment, light is launched into the cables 36 and 38, and is returned along the common cable 47 which is buried deeper than the cables 36 and 38, as is shown in Figure 12. Once again, the junction box 45 is connected to controller 47 which contains the pigtail laser diode light source for launching the light into the cables 36 and 38, and the detector for detecting return light to the cable 47.

As in the earlier embodiments, this embodiment could be used in a single multi-strain sensor mode in which a single waveguide in the cables 36 and 38 provides the two arms of the multi-strain sensor, and a waveguide in the cable 47 provides a common return path for the light received from the waveguides in the cables 36 and 38. However, the system can also be used in a dual sensor configuration using additional waveguides in the respective cables if desired.

This embodiment provides the maximum sensitivity of any of the embodiments disclosed because of the amount of cable which is used per unit length of the perimeter barrier, but is the most expensive to install because of the need to hand lay all of the cables and also the amount of cable which is utilised. However, in practice, this system permits very few null points because of the configuration of the three cables, as shown in the drawing in which the return cable 47 is out of phase with the other cables, and therefore this system greatly diminishes the possibility of someone breaching the perimeter barrier by standing on null points in the system. This embodiment therefore has greatest application for maximum security installations.

Since modifications within the spirit and scope of the invention may readily be effected by persons skilled within the art, it is to be understood that this invention is not limited to the particular embodiment described by way of example hereinabove.

Claims

1. A below ground security system comprising: a first cable having at least one first waveguide which forms at least part of a sensor configuration for detecting a breach of the security system; the first waveguide being buried below ground level; and a fill above the cable, and beneath which the cable is buried, the fill comprising a gravel or pebble fill.

2. The system of claim 1 wherein the system includes a second cable having at least one second waveguide, the first waveguide and second waveguide forming arms of a micro-strain sensor configuration.

3. The system of claim 1 wherein the at least one waveguide forms the sensor and operates in motion vibration sensor configuration.

4. The system of claim 2 wherein the first and second waveguides form two arms of a sensor so that a disturbance of one of the waveguides relative to the other changes a characteristic of light propagated through one of the waveguides, so as to change an interference pattern created when light from the first and second waveguides is recombined, to thereby provide an indication of a breach of the security system.

5. The system of claim 4 wherein the system includes a light source for launching light into the waveguides, and a detector for detecting light from the waveguides.

6. The system of claim 1 wherein the sensor is a micro-strain sensor.

7. The system of claim 1 wherein at least three cables are provided, each cable having at least one waveguide.

8. The system of claim 7 wherein one of the waveguides is a common waveguide and the two other waveguides comprise separate waveguides for providing two arms of a micro-strain sensor, with said one of the arms returning recombined light from both said two arms which form the micro-strain sensor.

9. The system of claim 7 wherein the at least two waveguides are laid in a zigzagged pattern relative to one another.

10. The system of claim 8 wherein a further cable including at least one waveguide is provided, the further cable forming a motion vibration sensor which is used alongside the micro-strain sensor.

11. The system of claim 7 further comprising a pair of waveguides arranged in a curved or undulating pattern into which light is launched, and at least one return waveguide for returning light from the said pair of waveguides, the at least one return waveguide being laid below the depth of the said two waveguides .

12. The system of claim 11 wherein the at least one return waveguide is laid in a straight line.

13. The system of claim 11 wherein said at least one return waveguide is also laid in a curved or undulating pattern.

14. A below ground security perimeter system, including: a first cable having at least one waveguide laid in a substantially straight line below ground level; a second cable having at least one waveguide laid in a substantially straight line below ground level; a light source for launching light into the first and second waveguides; at least one return waveguide for returning recombined light which has travelled through the first and second waveguides; and a detector for detecting light returned by the at least one return waveguide and for monitoring a parameter of the light to determine a breach of the security system.

15. The system of claim 14 wherein two return waveguides are provided.

16. The system of claim 13 or 14 wherein the or each return waveguide is provided in a separate cable.

17. The system of claim 15 wherein the two return waveguides may form a continuation of the first and second waveguides so that the first and second waveguides form twin arms of a single micro-strain sensor.

18. The system of claim 14 wherein the end of the first and second waveguides and the at least one return waveguide remote from the light source are coupled by a coupler for combining the light after the light has been transmitted through the waveguides. This configuration provides twin micro-strain sensors.

19. A below ground security perimeter system, including; a first cable having a first waveguide; a second cable having a second waveguide, the first and second cables being laid in a curved configuration and forming a micro-strain sensor; a third cable having a third waveguide and forming a motion vibration sensor; the first, second and third cables being buried below ground level; a light source for launching light into the first, second and third waveguides; and a detector for detecting light received from the first, second and third waveguides, and for detecting a change in parameter of light launched into the waveguides to thereby detect a breach of the security perimeter.

20. A below ground security perimeter system, including: a first cable having a first waveguide; a second cable having a second waveguide; the first and second cables being laid in a curved or undulating configuration; a light source for launching light into the first and second waveguides; at least one third waveguide for returning recombined light which has travelled through the first and second waveguides; a detector for detecting light returned by the return waveguide; the first, second and third waveguides being buried below ground level; and the first and second waveguides forming two arms of a micro-strain sensor.

21. The system of claim 20 wherein the return waveguide is laid in a straight line.

22. The system of claim 21 wherein the return waveguide is also laid in a curved or undulating configuration similar to that of the first and second waveguides. This configuration also avoids the possibility of null points in the system.